1. Field of the Invention
This invention relates to a magnetic material amount detecting apparatus which detects a small amount of magnetic material contained in a magnetic ink to be printed on sheets of paper such as securities, for example.
2. Description of the Related Art
A forgery prevention measure using a magnetic material is currently in wide use for bills and securities. For example, printing is done on bills or securities by use of a magnetic ink. Further, some bills or securities have a long, narrow strip-form magnetic material watermarked therein. Truth determination is made for the above sheets of paper by using the magnetic material amount detecting apparatus to detect the magnetic ink or strip-form magnetic material in the distribution stage thereof.
The magnetic material amount detecting apparatus which carries out truth determination for the above sheets of paper has a magnetic head to detect the magnetic material in the sheet of paper. In the above magnetic material amount detecting apparatus, a magnetic head of a differential winding type transform system, D.C. excitation system or impedance system is used.
In the magnetic head of the differential winding type transform system, a primary winding is mounted on the central portion of an S-shaped core and secondary windings are wound on portions thereof which are set near the small gaps on the two opening sides. In the magnetic material amount detecting apparatus using the magnetic head with the above configuration, a magnetic material contained in a sheet of paper which is passed near one of the opening portions of the magnetic head is detected by use of a difference between induced voltages caused by the two secondary windings.
In the magnetic head of the D.C. excitation system, a small gap is formed in part of an annular core on which primary and secondary windings are mounted. In the magnetic material amount detecting apparatus using the magnetic head with the above configuration, a D.C. current is supplied through the primary winding and a variation in the magnetic flux caused in the annular core when a magnetic material passes on the gap formed in the magnetic head is sensed based on induced voltage of the secondary winding.
In the magnetic head of the impedance system, a small gap is formed in part of an annular core. The magnetic head of this type is used to detect a variation in the magnetic flux caused in the annular core when a magnetic material passes over the gap as a variation in the impedance of the winding wound on the core by use of an A.C. bridge circuit. Further, there is provided a method for applying a magnetic bias to a magneto-resistance element by use of a permanent magnet and detecting a variation in the magnetic field caused when the magnetic material is set closer to or separated apart from the magneto-resistance element as a variation in the resistance thereof.
However, in the magnetic material amount detecting apparatus using the magnetic head of the differential winding type transform system, it is necessary to obtain a signal which varies in proportion to a magnetic material amount while compensating for a variation in the magnetic permeability due to a temperature variation. Therefore, the magnetic material amount detecting apparatus using the magnetic head of the differential winding type transform system has a disadvantage that the cost becomes high because a plurality of cores and windings are necessary and impedances on the detecting side and compensating side are required to be adjusted.
Further, in the magnetic material amount detecting apparatus using the magnetic head of the impedance system, it is required to combine two sets of magnetic heads in order to temperature-compensate the core. Therefore, the magnetic material amount detecting apparatus using the magnetic head of the impedance system has a disadvantage that the cost becomes as high as the magnetic material amount detecting apparatus using the magnetic head of the differential winding type transform system.
In the magnetic material amount detecting apparatus using the magnetic head of the D.C. excitation system, an output signal derived from the magnetic material varies in proportion to the traveling speed of the magnetic material. Therefore, the magnetic material amount detecting apparatus using the magnetic head of the D.C. excitation system has a disadvantage that a detected signal is not always set equal to a signal value which varies in proportion to the magnetic material amount.
Further, the magnetic material amount detecting apparatus using the magnetic head of the magneto-resistance system has a structure in which two magneto-resistance elements are arranged on the same plane in order to reduce the influence of a temperature drift and a difference in the magnetic strength between the two elements is output as a signal. Thus, in the magnetic material amount detecting apparatus using the magnetic head of the magneto-resistance system, a difference between the amounts of spatial magnetic materials on the two elements is derived instead of the magnetic material amount. That is, the magnetic material amount detecting apparatus using the magnetic head of the magneto-resistance system has a disadvantage that a signal indicating the precise magnetic material amount is not derived.